1. Field of the Invention
This invention relates to radio frequency and wireless communications and more particularly relates to how a radio frequency device can serve to locate a transceiver.
2. Description of the Related Art
In order for Radio Frequency (RF) devices to be useful, there must be a means of superimposing information on a transmitted radio wave and subsequently extracting the information from a received radio wave. Typically, an RF transmitter selects a particular frequency as a carrier and modulates information on the carrier. An RF receiver usually tunes a selection filter to the carrier frequency and detects information by determining a change of frequency or amplitude of the carrier. It is well known that if multiple transmitters select the same frequency as a carrier, signal interference will result. This interference often makes detection of information difficult or impossible. Uncontrolled access to the radio frequency spectrum would flood the airways with interference and ultimately make radio frequency devices unusable.
For this reason, international treaties were signed that require national governments around the world to regulate and control public access to the radio frequency spectrum. Free access to the radio spectrum is only allowed at extremely low power levels to insure that transmitters operated by the general public do not interfere with other radio frequency devices that could be on the same or adjacent frequencies.
Rules and regulations are clear about what types of devices and what types of communications are allowed on each frequency band. For example, there are two citizen bands (CB) in the United States of America where devices can be used for public communications. CB radios are often used by truck drivers as they travel throughout the United States. Walkie-talkies in these bands may be used by a parent or guardian to maintain contact with a child old enough to understand how to use such a device.
The general public is authorized to freely use many frequency bands if the transmission power level is limited to a low power. Garage door openers and automobile door lock controls are examples of low power public use of the RF spectrum. Because of the mandated RF transmitter power restrictions, the useful range of these public-use devices is severely limited. Licenses may be obtained to use higher power transmitters in certain frequency bands. For example, amateur radio (HAM) licenses are granted based upon passing certain tests and demonstrating proficiency in certain communication skills.
Location using RF communications has been practiced for many years. For example, research has been conducted on the migratory habits of birds by using RF transmitters attached to the birds and RF receivers equipped with directional antennas. Generally, these devices transmit on a regulated frequency band. Difficulties with location tracking using RF devices are generally associated with the mobile transmitter range. If a receiver cannot receive communication due to distance, obstructions, or interference, then bearing and distance cannot be established. The range of a tracking device is typically dependent on transmitter power, and transmitter power is generally dependant on an available power source and applicable government regulations for the selected frequency band.
For example, when a mobile transmitter is miniaturized to permit attachment to a child or small animal, the limited life of a small battery may limit the number of transmission messages and transmission power level. A tradeoff is usually made between decreasing the transmission power to increase battery life and increasing the transmission power to the extent possible under applicable regulations, to extend the transmit range.
FIG. 1 is a schematic block diagram illustrating a typical prior art RF tracking system 100. The RF tracking system 100 includes a mobile RF device 110 and a companion RF device 140. An obstruction 170 may prevent line-of-sight communication between the mobile RF device 110 and the companion RF device 140.
In a simple system, the mobile RF device 110 may be a transmitter with transmit range 120 and the companion RF device 140 may be a receiver with reception range 160. A transmit range 120 may be dependent on transmitter power, antenna design, and the like while the reception range 160 may be dependent on interference, obstructions 170, antennae design, and so forth. The interaction between the transmit range 120 and the reception range 160 is called the communication range. The companion RF device 140 may track the location of the mobile RF device 110 if the communication range spans the separation distance between the devices. An obstruction 170 may decrease the communication range. It is desirable to increase the transmit range 120 of the mobile RF device 110 as much as possible.
A difficulty with the simple RF tracking system 100 is that the mobile RF device 110 may be unaware whether the companion RF device 140 is within communication range. In the simple RF tracking system 100, the mobile RF device 110 may transmit continuously in an attempt to communicate with the companion RF device 140 until the mobile power source is exhausted. The transmission power may initially be high due to a fully charged power source and gradually decrease as the power source becomes discharged. If the companion RF device 140 does not locate the mobile RF device 110 or enter the communication range before the power source is exhausted, the system 100 fails. Decreasing the transmit range 120 by decreasing the transmit power may increase the available transmission time, but may also decrease the probability that the companion RF device 140 is within communication range.
In a more complex system, the mobile RF device 110 may be a transceiver with transmit range 120 and reception range 130, and the companion RF device 140 may be a transceiver with transmit range 150 and reception range 160. The companion RF device 140 may track the mobile RF device 110 by transmitting a signal to the mobile RF device 110, which in turn transmits a signal with location information to the companion device 140. An advantage of this approach is that the mobile RF device 110 may conserve power by waiting to transmit until it is likely that the companion RF device 140 is within communication range.
A complexity introduced with the more complex system is that the communication between the companion RF device 140 and the mobile RF device 110 should work in both directions. The companion RF device 140 may not have the space and weight constraints associated with the mobile RF device 110, and may therefore contain a power source with greater capacity and a transmitter with a greater transmit range 150 than the mobile RF device 110. In this case, the mobile RF device 110 may receive a message and transmit a response that is not received by the companion RF device 140 because of the limited transmit range 120.
A difficulty with unlicensed public use of this more complex system using low power transmission is that the transmit ranges 120, 150 may be limited to a few hundred feet. Additionally, receiving messages on a frequency band used by other traffic often leads to RF interference and unwanted messages, and may necessitate inclusion of an identifying code and use of a technique to verify the validity of a message.
The wireless telecommunications infrastructure may also be used for radio tracking. The existing radio allocations, which have enough power to be effective, authorize wireless companies to create and maintain various wireless telecommunications infrastructures. Devices used in these infrastructures include, but are not limited to: cellular telephones, two-way pagers, and devices that use the general packet radio service.
One system uses a specialized cellular telephone that has a built-in global positioning system receiver. A user can call the cell phone number issued by the telephone company, and the specialized cell phone transmits the GPS coordinates of the device. For use in location tracking of people, the device can be disguised as a watch or can be otherwise concealed. Unfortunately, such specialized devices are very expensive, often beyond the reach of those who could most use such technology. The cost of location systems using the cellular telephone network may be not only the purchase of the device, but also may be a recurring monthly service charge. In addition, wireless telecommunications service may not be reliable when the cell phone is within a structure or the signal is otherwise obstructed, and the wireless service may not be available in rural and/or in remote areas.
What is needed is an apparatus, method, and system that can locate a transceiver by using RF communications and radio services. Use of RF communications and radio services for location monitoring may eliminate a cellular telephone recurring charge, and may be available in all parts of the country. The apparatus and system needs to be available for use by the unlicensed general public within the limits of government regulations, and the transmission range should be extended well beyond a few hundred feet. The apparatus needs to conserve energy, providing a long effective life without requiring a large power source. The apparatus and system needs to be effective in spite of RF interference and physical obstacles that attenuate RF signals. Additionally, the tracking system needs to be inexpensive, such that all segments of society can afford to own and use it. Moreover, the system should not require a large infrastructure requiring constant maintenance by paid employees.